The present invention relates to a glass substrate for an optical disk such as a digital audio disk (so-called compact disk) and a magneto-optical disk, and more particularly to a glass substrate for an optical disk to be manufactured by a so-called 2P method (photopolymerization method) in which a rugged pattern of pits and grooves is transferred onto the glass substrate by photopolymerization of ultraviolet hardening resin.
In manufacturing an optical disk such as a digital audio disk and a magneto-optical disk, a transfer process is carried out to transfer a rugged pattern of pits, such as for an address signal and guide grooves onto a disk substrate. As such a transfer process, there is proposed a so-called 2P method such that a stamper on which the rugged pattern is formed is brought into close contact with liquid ultraviolet setting resin, and ultraviolet light is irradiated to the ultraviolet setting resin to photopolymerize the same.
According to the 2P method, the liquid ultraviolet setting resin is applied to one surface of the disk substrate having a light transmissivity by screen printing, for example, and the stamper on which the rugged pattern is formed is brought into contact with the ultraviolet setting resin applied onto the surface of the disk substrate. Then, ultraviolet light is irradiated to the ultraviolet setting resin from the side of the disk substrate, thereby photopolymerizing the ultraviolet setting resin to harden the same. Thereafter, the disk substrate with a layer of the ultraviolet setting resin thus hardened is separated from the stamper. As a result, the fine rugged pattern (pits and grooves) formed on the surface of the stamper is transferred to the surface of the disk substrate.
As mentioned above, the ultraviolet setting resin in a liquid state is brought into close contact with the stamper according to the 2P method. Therefore, it is advantageous that a high fidelity of duplication can be obtained and that the stamper is hardly deteriorated. Particularly in the case that a glass substrate is used as the disk substrate, it is greatly advantageous from the viewpoints of dimensional stability, heat resistance and low double refraction.
In applying the ultraviolet setting resin to the entire surface of the glass substrate, there is a possibility of spewing of the resin or generation of bubbles. Accordingly, the application of the ultraviolet setting resin is carried out using screen printing employing a printing pattern set so as to ensure uniform spread of the resin and eliminate the problem of spewing, etc.
However, even though the ultraviolet setting resin is applied with the desired printing pattern for the purpose of preventing the spewing of the resin or the generation of the bubbles, there is a possibility that an excess part of the ultraviolet setting resin is spewed from inner and outer circumferential portions of the glass substrate in the step of bringing the resin into close contact with the stamper. If such a spewed part of the resin is left as it is, it will be hardened by the ultraviolet irradiation in the next step. As a result, the spewed resin thus hardened remains as a bur. The generation of such a bur hinders recording and reproduction particularly in case of a magnetooptical disk to be accommodated in a cartridge, thus causing a reduction in yield.